Its fairly complicated, so I'll point you to the all-powerful wikipedia entry, and the most layman howstuffworks, respectfully.
http://en.wikipedia.org/wiki/Dsl
http://electronics.howstuffworks.com/dsl.htm
Quote:
To understand DSL, you first need to know a couple of things about a normal telephone line -- the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second -- see How Telephones Work for a great demonstration of this). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases.
The use of such a small portion of the wire's total bandwidth is historical -- remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity. DSL does just that.
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The main thing in DSL speed is how close you reside to the central office, or to a routing station. The closer, the faster; the futher, the slower; whereas cable its generally equal everywhere. If you can find out how close you live to a routing station, you could find out if it is slower or faster than cable.
That is, if they offer it in your area.